Variable Speed of Light from Planck-Scale Spacetime Discretisation: Cosmological Implications for the Horizon Problem, Dark Energy, and the Hubble Tension

We explore the cosmological consequences of a variable speed of light (VSL) motivated by Planck-scale spacetime discretisation. If spacetime is quantised at the Planck length — as proposed in Loop Quantum Gravity, Causal Set Theory, and the holographic principle — the causal signal propagation rate c may vary as the density of Planck-volume cells evolves. We demonstrate that c decreasing from a near-infinite value at the cosmological singularity produces two rigorously derived results: the particle horizon diverges logarithmically, resolving the horizon problem without an inflaton field; and supernova light curve time dilation of (1+z) follows exactly from the light travel integral without metric expansion. We further show that the hydrogen binding energy is exactly c-independent, preserving the recombination epoch. Fitting the Pantheon+ Type Ia supernova dataset (1,573 SNe Ia) with the static VSL luminosity distance formula suggests the VSL mechanism may partially account for distances currently attributed to dark energy, and the framework provides a candidate mechanism for the Hubble tension (H0_local/H0_CMB discrepancy), operating in the correct direction with the correct order of magnitude. Six testable predictions are identified, including dark energy equation of state w(z) ≠ −1 (testable with Euclid), fine structure constant variation at z < 0.4 (testable with ELT/ANDES), and gamma-ray burst time delay z-dependence (testable with existing Fermi data). This work is presented as a theoretical exploration and direction of travel rather than a complete alternative cosmology; key open problems are identified throughout.